What Are the Design Basis for Injection Molds?

In injection molding[^1], mold design directly determines product quality, production efficiency, and manufacturing costs. Below is a comprehensive breakdown of the core design principles, procedures, and key systems that guide professional injection mold development.

Core Design Basis

1. Dimensional Accuracy & Correctness of Related Dimensions

The external quality and specific dimensions of plastic products are defined by their functional requirements:

• Appearance-focused products (e.g., toys): High aesthetic standards but moderate dimensional precision.
• Functional products: Strict dimensional tolerances to ensure performance.
• High-precision products (e.g., cameras): Stringent requirements for both appearance and dimensions.

2. Rationality of Draft Angle[^2]

The draft angle is critical for smooth demolding and product integrity:

• Ensure sufficient draft to prevent sticking during ejection.
• Align the angle with the parting/split mold surface.
• Avoid compromising appearance or wall thickness accuracy.
• Prevent weakening of critical structural areas.

Standard Design Procedure

1. Analysis of Product Drawings & Physical Samples

Thoroughly evaluate the product to establish design constraints:

• Geometric shape and structural complexity.
• Dimensional specifications, tolerances, and design datums.
• Technical requirements (e.g., strength, flexibility).
• Plastic material grade and surface finish demands.

2. Cavity Quantity & Layout

Determine the number of cavities based on:

• Product weight vs. injection machine capacity.
• Projected area vs. clamping force.
• Mold size vs. machine’s tie-bar spacing.
• Product precision, color, and production volume.
• Economic efficiency (output value per mold).

Cavity Layout Considerations:

• Optimize mold size and gating system balance.
• Coordinate with core-pulling mechanisms, inserts, and cooling systems.
• Adjust based on parting surface and gate position selection.

Key Design Elements

1. Parting Surface Selection

The parting surface should:

• Preserve product appearance and precision.
• Simplify mold machining (especially cavity fabrication).
• Facilitate gating, exhaust, and cooling system integration.
• Ensure the product remains on the moving mold during demolding.
• Accommodate metal inserts easily.

2. Gating System Design

The gating system controls plastic flow into the cavity. Key steps:

1. Gate Position Selection (follow these principles):

• Locate on the parting surface for easy cleaning.
• Ensure equal distance to all cavity areas (shortest flow path).
• Direct flow toward thick-walled sections for smooth filling.
• Avoid impinging on cores/inserts (prevents deformation).
• Minimize weld lines or place them in non-critical areas.
• Enable uniform filling and efficient gas evacuation.
• Position for easy post-processing without affecting appearance.

2. System Components:

• Sprue: Main channel connecting the injection machine to the mold.
• Runner: Distributes plastic to multiple cavities (optimize cross-section for flow).
• Gate: Controls flow rate and pressure (e.g., pin-point, edge, or sub-gates).
• Degating Mechanism: For pin-point gates, ensure runner separation during ejection.

3. Exhaust System Design

Effective venting prevents defects like air traps and burn marks:

• Vent Grooves: Position at the last-filled cavity areas. Depth varies by material:
  • ABS: ≤0.04 mm
  • Putty: ≤0.02 mm
  • POM (Delrin): ≤0.02 mm
• Clearance Venting: Use gaps between cores, ejector pins, or special exhaust plugs.
• Air Pins: Prevent vacuum deformation during product ejection.
• Anti-Vacuum Components: Avoid product adhesion to the mold surface.

4. Cooling System Design[^3]

Uniform cooling ensures consistent product quality and cycle time reduction. Design considerations:

• Cooling channel layout (e.g., parallel, series, or spiral).
• Position and size of channels (avoid interfering with other mold components).
• Targeted cooling for high-heat areas (e.g., cores, inserts, sliders).
• Selection of standard cooling components (pipes, baffles, O-rings).
• Sealing structure to prevent water leakage.

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  > Note > > Injection mold design is a iterative process—each system interacts with others. Balancing functionality, manufacturability, and cost requires careful adjustment based on specific product requirements.

  [^1]: Explore this resource to gain in-depth knowledge about injection molding, enhancing your design and production efficiency. [^2]: Exploring this will help you grasp how draft angles influence product integrity and demolding efficiency. [^3]: Effective cooling design is crucial for maintaining product quality and reducing cycle times.

About the Author

Alex Yu

Ideal Pro Manufacturing

Alex is the co-founder of Ideal Pro, but when he's not overseeing key projects or steering the technical team, you'll likely find him on a long bike ride, hunting for the best coffee in town, or planning his next travel adventure. With over 20 years of experience spanning international trade sales and project management, he's seen it all — tight deadlines, tricky engineering problems, and everything in between. He believes great work comes from a well-rested mind and a good cup of coffee, and he brings that same curiosity and attention to detail to every project he takes on. When he's not in the office, he's usually somewhere outdoors, exploring new places and collecting stories to share.